We recently described the enrichment of APOBEC3 mutagenesis later in tumour evolution, occurring as subclonal mutations in estrogen receptor (ER)-negative breast cancer, lung adenocarcinoma, head and neck squamous carcinoma and bladder carcinoma, suggesting APOBEC3 may contribute to branched evolution in some tumour types [12–14]. This evidence concerns the gene ESR1 and lung adenocarcinoma.